Receiver for receiver a signal transmitted by a time-division multi-access method

ABSTRACT

A receiver receives a signal transmitted by a time-division multi-access method that divides one frame into a plurality of communication slots in conducting communication. In this receiver, a detection circuit detects whether a received RF signal is stronger than a predetermined level. The received RF signal is demodulated by a demodulation circuit. An oscillation circuit generates a clock that is used to decode the data obtained by demodulation. The data obtained by demodulation is stored in a memory in synchronism with a clock reproduced from the received signal, and is then retrieved from the memory in synchronism with the clock generated by the oscillation circuit. This eliminates jitters. After jitter elimination, the data is decoded by a processing circuit. The writing/reading operation against the memory is initialized in accordance with a result output from the detection circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a receiver for receiving a signaltransmitted by a time-division multi-access (TDMA) method as used in acellular telephone network like the Personal Handyphone System(hereinafter “PHS”.

2. Description of the Prior Art

In a receiver (portable terminal) for receiving a signal transmitted bya time-division multi-access method as used in the PHS, the clock signalthat is transmitted together with the data signal from a base station soas to be used for the demodulation of the data signal and the clocksignal that is generated within the receiver for the processing of thedemodulated data signal are asynchronous with respect to each other andhave different frequencies. This makes it inevitable that thedemodulated data signal is processed by a processing circuit (forexample, a TDMA protocol circuit) that operates asynchronously withrespect thereto, and thus causes jitters.

For this reason, a conventional receiver is provided with a jitterelimination circuit as shown schematically in FIG. 9. This jitterelimination circuit employs a FIFO (first-in-first-out) memory 60. Thereproduced data RD1 and the reproduced clock RCLK that are obtainedthrough demodulation of a received signal by a demodulator are fed tothe memory 60 so that the reproduced data RD1 is stored in the FIFOmemory 60 in synchronism with the reproduced clock RCLK. Then, thereproduced data is retrieved as RD2 from the FIFO memory 60 insynchronism with the clock CLK output from an oscillator provided withinthe receiver.

Since the frequency of the reproduced clock RCLK, which is areproduction of the clock transmitted from the base station, isdifferent from that of the clock output from the oscillator, thereceived data is first stored in the FIFO memory 60 in synchronism withthe operation timing of the transmitting side, and is then retrievedtherefrom in synchronism with the operation timing of the receivingside. This helps eliminate jitters. The retrieved data RD2 is thensubjected to signal processing, such as waveform shaping and decoding,performed by a TDMA protocol section. In a case where the receiver is aPHS receiver, the processed data is output as sounds from a loudspeakerunder the control of a CPU (central processing unit).

In a conventional receiver, the writing/reading operation against theFIFO memory 60 is reset for restarting (i.e. initialized) at the end ofa receiving session, and therefore the writing/reading operation lastsfor a comparatively long time interval (5 milliseconds) that correspondsto one frame in PHS communication. On the other hand, as describedpreviously, the clock on the transmitting side (i.e. the transmittedclock) and the clock generated by an oscillation circuit on thereceiving side have different frequencies. The difference in timingbetween these two clocks (i.e. the phase difference between the twoclocks) becomes larger as time passes. This requires that, in the jitterelimination circuit shown in FIG. 9, the writing operation and thereading operation be performed at increasingly distant locations fromeach other, and accordingly that the FIFO memory 60 have an unduly largestorage capacity as achieved by the use of, for example, RAM1 to RAM10.Note that RAM1 to RAM10 each correspond to, for example, one bit ofdata.

Moreover, a large loss of time arises between the time point when thereproduced data RD1 is stored in the FIFO memory 60 and the time pointwhen it is retrieved as the reproduced data RD2, and this causes anundesirable delay in the processes performed by the receiver, forexample, for achieving synchronism with the received signal and forsearching for the base station that offers the best communicationquality.

SUMMARY OF THE INVENTION

An object of the present invention is, in a receiver for receiving asignal transmitted by a time-division multi-access method, to minimizethe storage capacity of the memory used by a jitter elimination circuitand minimize the delay from reception of a signal until starting ofsignal processing.

To achieve the above object, according to one aspect of the presentinvention, a receiver for receiving a signal transmitted by atime-division multi-access method is provided with: a signal strengthdetection circuit for detecting whether the signal strength of a signaltransmitted by a time-division multi-access method is stronger than apredetermined level; a demodulator for demodulating the signal to outputreproduced data and a reproduced clock; an oscillator for outputting aninternal clock; a memory to which the reproduced data is written insynchronism with the reproduced clock and from which the reproduced datais read in synchronism with the internal clock; and an initializationcircuit for initializing the timing with which the reproduced data iswritten to and read from the memory.

According to another aspect of the present invention, a receiver forreceiving a signal transmitted by a time-division multi-access methodthat divides one frame into a plurality of communication slots inconducting communication is provided with: a detection circuit fordetecting whether a received RF signal is stronger than a predeterminedlevel; a demodulation circuit for demodulating the received RF signal toobtain therefrom reproduced data and a reproduced clock; an oscillationcircuit for generating an internal clock; a jitter elimination circuitfor eliminating jitters from the reproduced data by writing thereproduced data to a memory in synchronism with the reproduced clock andreading the reproduced data from the memory in synchronism with theinternal clock; a processing circuit for decoding the reproduced dataafter jitter elimination; and an initialization circuit for initializingthe jitter elimination circuit in accordance with a result output fromthe detection circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of the present invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanying drawings inwhich:

FIG. 1 is a block diagram of a receiver embodying the invention;

FIG. 2 is a block diagram showing the elements provided within andaround the signal strength detection circuit incorporated in the RFcircuit of the receiver of the embodiment;

FIG. 3 is a diagram showing the frame structure of the signal used inthe PHS;

FIG. 4 is a diagram showing the relationship among the frame clock, thebit clock, and the reproduced data in the receiver of the embodiment;

FIG. 5 is a diagram illustrating the operation of the signal strengthdetection circuit of the receiver of the embodiment;

FIG. 6 is a block diagram of the jitter elimination circuit of thereceiver of the embodiment;

FIG. 7 is a diagram showing the relationship between the write vectorsignal and the read vector signal output from the counter of the jitterelimination circuit shown in FIG. 6;

FIG. 8 is a waveform diagram illustrating the operation of the jitterelimination circuit shown in FIG. 6; and

FIG. 9 is a diagram schematically showing the jitter elimination circuitprovided in a conventional receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be describedwith reference to the accompanying drawings. FIG. 1 is a block diagramof a receiver embodying the invention. In this receiver, an antenna 1receives a signal transmitted from a base station by a time-divisionmulti-access method, and an RF (radio frequency) circuit 2 performsprocessing, such as RF amplification, on the received signal. As will bedescribed later, the RF circuit 2 has a signal strength detectioncircuit 10 (see FIG. 2), with which it detects the signal strength ofthe received RF signal (i.e. the intensity of the electric fieldproduced by the RF signal). In accordance with the detected signalstrength, the RF circuit 2 outputs a reset signal RESET.

The RF signal output from the RF circuit 2 is demodulated by ademodulation circuit 3, which thereby outputs reproduced data RD1 and areproduced clock RCLK. The jitters occurring between the reproducedclock RCLK and the clock output from an oscillation circuit 5, such ascaused by the phase difference between these two clocks, are eliminatedby a jitter elimination circuit 4. To achieve this, the jitterelimination circuit 4 stores the reproduced data RD1 to a FIFO memory 42(see FIG. 6) provided therein in synchronism with the reproduced clockRCLK, and outputs it as reproduced data RD2 to a TDMA protocol section 6provided in the next stage in synchronism with the clock CLK from theoscillation circuit 5.

On receiving the reproduced data RD2 from the jitter elimination circuit4, the TDMA protocol section 6 performs waveform shaping and decoding onit in synchronism with the clock CLK from the oscillator 5, and outputsthe decoded data. This data is stored in a memory 7. The data stored inthe memory 7 is read by a CPU 8 and is output as sounds from aloudspeaker 9. Although no illustration is provided, it is also possibleto output the data through an interface to a personal computer connectedto the receiver.

FIG. 2 is a block diagram showing an example of the elements providedwithin and around the signal strength detection circuit 10 incorporatedin the RF circuit 2 (see FIG. 1). Within the signal strength detectioncircuit 10, the RF signal received by the antenna 1 is first fed to anenvelope detector 11 to extract the amplitude component thereof, and isthen fed to an A/D converter 12 so as to be converted into a digitalsignal. This digital signal is integrated by an integrator 13, and isthen output as a signal strength detection signal RSSI from the signalstrength detection circuit 10. The signal RSSI is compared with apredetermined threshold voltage by a comparison circuit 14, and theresult of comparison is output as the reset signal RESET from the RFcircuit 2.

FIG. 3 shows the frame structure of the signal used in the PHS, takingas examples the first and second frames F1 and F2. One frame lasts for 5milliseconds. Each of the frames F1 and F2 consists of an interval Tx inwhich the base station transmits data and the like to the receiver andan interval Rx subsequent thereto in which, in exchange, the receivertransmits data and the like to the base station. The intervals Tx and Rxare each subdivided into four intervals (T1 to T4 and R1 to R4). Each ofthese subdivided intervals is called a slot. Within each of theintervals Tx and Rx, one slot is allotted to one receiver (portableterminal). Accordingly, in the example shown in the FIGURE, a singleframe permits four receivers to communicate with the base station.

For example, when the base station is communicating with a receiver, thereceiver receives the signal addressed thereto for an interval of 625microseconds that is allotted thereto, and receives other signals duringthe other intervals. Accordingly, although the receiver theoreticallyneeds to eliminate a deviation of ±4 bits of the bit clock within oneframe as described previously, in this case, where the interval in whichthe receiver actually receives the signal occupies one slot that lastsfor a time interval as short as 625 microseconds, it can safely be saidthat the receiver has only to eliminate a deviation of ±½ bit of the bitclock. For this reason, in this embodiment, as will be described later,the jitter elimination circuit 4 (see FIG. 1) is initialized at thestart of a frame.

In the present specification, the frame clock and the bit clock aredefined as follows. In FIG. 4, at (a) is shown the frame clock, at (b)is shown the bit clock, and at (c) is shown the data. One slot consistsof 240 bits, and therefore the bit clock has a frequency of 240 bits/625microseconds =384 kHz. The data is refreshed in synchronism with risingedges of the bit clock. The frame clock has a period twice the period ofthe bit clock, and therefore the data is refreshed every time the frameclock changes its level.

FIG. 5 shows the relationship between the signal strength detectionsignal RSSI output from the signal strength detection circuit 10 (seeFIG. 2) and the threshold voltage Vref. When the receiver is receivingno signal, as in the interval W1, the signal strength detection signalRSSI is lower than the threshold voltage Vref.

When the receiver receives a signal, as in the subsequent interval W2,the signal strength detection signal RSSI starts rising and then, at thetime point t0, exceeds the threshold voltage Vref. At this time, thecomparison circuit 14 (see FIG. 2) outputs the reset signal RESET to thejitter elimination circuit 4, and thereby the jitter elimination circuit4 is initialized. In the subsequent interval W3 after the end of theinterval W2, the signal strength detection signal RSSI quickly dropsuntil it becomes lower than the threshold voltage Vref.

FIG. 5 shows a case in which the receiver receives a signalcorresponding to one slot in the interval W2. In the interval W2, theslot is divided into intervals A and B. The first interval A chieflyserves as a preamble to synchronize the received signal with theprocessing performed in the receiver. The next interval B is chieflyused to communicate data such as sounds. In this embodiment,initialization is performed at the start of a slot, and therefore onlysmall jitters are left to be eliminated at the end of the slot shown bynumeral 20 and arrows.

FIG. 6 is a block diagram showing the internal configuration of thejitter elimination circuit 4. The jitter elimination circuit 4 includesa FIFO memory 42 having three memories for storing the reproduced dataRD1, a counter 40 for generating vector specification signals CUNT1 andCUNT2 used to specify the vectors (addresses) at which data is writtento and read from the FIFO memory 42, a delay circuit 41 for generating aFIFO start flag signal RFLG, a latch circuit 43 for generating a timingsignal RFLG0, and a latch circuit 44 for latching the reproduced dataFOUT. On receiving the reset signal RESET from the RF circuit 2 (seeFIG. 1), the jitter elimination circuit 4 is reset; thus, the counter 40and the FIFO memory 42 are initialized. CUNT1 is used when data RD1 iswritten to the FIFO memory 42, and CUNT2 is used when data RD1 is readfrom the FIFO memory 42.

The reproduced data RD1 is stored in the FIFO memory 42 in synchronismwith the reproduced clock RCLK The three memories constituting the FIFOmemory 42 will be referred to as FIFO(0), FIFO(1), and FIFO(2). Thecounter 40, which operates on a two-bit basis, generates the vectorspecification signal CUNT1 that is used to specify the vector at whichthe reproduced data RD1 is stored. The counter 40 outputs the vectorspecification signal CUNT1 by counting the rising edges of thereproduced clock RCLK after resetting (initialization). As shown in FIG.7, at every rising edge of the reproduced clock RCLK, the value of CUNT1changes periodically like 0, 1, 2, 0, 1, 2, and so forth. The values 0,1, and 2 specify the memories FIFO(0), FIFO(1), and HFO(2),respectively.

The counter 40 generates also the vector specification signal CUNT2 thatis used to specify the vector at which the data FOUT (data RD1) isretrieved from the FIFO memory 42. The counter 40 outputs the vectorspecification signal CUNT2 by counting the 38 kHz clock CLK2 output fromthe oscillator 5. As shown in FIG. 7, the value of CUNT2 changesperiodically like 0, 1, 2, 0, 1, 2, and so forth. Note that the counter40 starts counting CLK2 not immediately after initialization, but aftera predetermined delay time produced by the delay circuit 41 and thelatch circuit 43.

The delay circuit 41 generates the signal RFLG that indicates thatstoring of the reproduced data RD1 to the FIFO memory 42 has beenstarted after resetting of the jitter elimination circuit 4 by the resetsignal RESET. The latch circuit 43 generates the signal RFLG0 from thesignal RFLG and the clock CLK1 and outputs the generated signal RFLG0.This signal RFLG0 indicates the timing with which retrieval of the datastored in the FIFO memory 42 should be started.

As shown in FIG. 8, after the signal RESET has turned to a high level,the counter 40 counts the reproduced clock RCLK to generate CUNT1. Onthe other hand, at the second trailing edge of the reproduced clock RCLKafter the signal RESET has turned to a high level, the delay circuit 41outputs RFLG. The latch circuit 43 generates RFLG0 in such a way thatRFLG0 turns to a high level at the first rising edge of the clock CLK1after RFLG has turned to a high level. When RFLG0 turns to a high level,the counter 40 counts the trailing edges of the clock CLK2 to output thevector specification signal CUNT2. The clocks CLK1 and CLK2 are bothgenerated by the oscillation circuit 5 shown in FIG. 1, and CLK1 has afrequency twice as high as the frequency of CLK2. By making thefrequency of CLK1 higher (i.e. by shortening its period), it is possibleto make RFLG0 turn to a high level more quickly after RFLG has turned toa high level. This makes it possible to start the reading operation asearly as possible.

In this embodiment, the jitter elimination circuit 4 is reset forrestarting (i.e. initialized) at the start of a PHS signal receptionsession. For example, in FIG. 3, consider a receiver (portable terminal)that communicates with the base station using the slots T1 and R1. Inframe F1, the jitter elimination circuit 4 is reset for restarting atthe time point α1 when a received signal is detected, and, in thesubsequent frame F2, it is reset for restarting again at the time pointα2 when a received signal is detected. In this case, the receiver as awhole starts operating at the time point α1 and keeps operating not onlyduring the interval of the slot T1 but continuously during thesubsequent slots T2, 73, T4, R1, R2, R3, and R4 even if no signal isreceived. The reason that the receiver continues the reception operationeven during the slots other than T1 is that where (i.e. in which slot)to receive a signal addressed thereto is not known in advance.

The phase difference between the clocks on the transmitting andreceiving sides increases as time passes like T1, 2, . . . , R4.However, the phase difference has no effect in the slots T2 and after,because these slots have nothing to do with the reception operation ofthe receiver in question. In other words, during the reception operationof this receiver (i.e. during the slot T1), there exists only a smallphase difference between the clocks on the transmitting and receivingsides. This means that jitters can be eliminated more easily, and thatthe FIFO memory 42 requires less storage capacity. By contrast, if thejitter elimination circuit 4 is reset for restarting at the time pointsβ1 and β2 shown in FIG. 3, the receiver needs to perform signalprocessing in the subsequent slot where it needs to perform thereception operation in the presence of a larger phase difference betweenthe two clocks. This means that jitters cannot be eliminated easily, andthat the FIFO memory requires more storage capacity.

In the above description, it is assumed that the signal received duringthe slot T1 is the signal addressed to the receiver in question. In acase where the signal received during the slot T3 is the signaladdressed to the receiver in question, the jitter elimination circuit isinitialized (i.e. reset for starting) at the start of the slot T3.

Back in FIG. 6, when the signal RFLG0 is at a high level, the latchcircuit 44 latches the data FOUT and outputs the data RD2 at the risingedges of the clock CLK2. The latch circuit 44 serves to perform waveformshaping on the data FOUT retrieved from the FIFO memory 42.

As described above, in the receiver of this embodiment for receiving asignal transmitted by a time-division multi-access method, the jitterelimination circuit 4 is reset at the start of a signal receptionsession by the use of the signal strength detection circuit 10 providedin the RF circuit 2. This makes it possible to eliminate jitters whilethe jitters are relatively small, and thus helps reduce the storagecapacity required in the memory provided in the jitter eliminationcircuit 4.

Moreover, by reducing the storage capacity of the FIFO memory 42, it ispossible to shorten the delay time that arises between the time pointwhen the jitter elimination circuit 4 stores the reproduced data RD1 andthe time point when it outputs it as the data RD2. This makes itpossible to quicken the processes performed by the receiver, forexample, for achieving synchronism with the received signal and forsearching for a base station.

In this embodiment, as shown in FIG. 6, both the counter 40 and the FIFOmemory 42 are initialized. However, the same effect can be obtained byinitializing the counter 40 and the latch circuit 43 withoutinitializing the FIFO memory 42. Moreover, although the PHS is taken asan example in the above description of the embodiment, it is possible,in principle with any signal transmitted by a time-division multi-accessmethod, to reduce the storage capacity of the memory by initializing thejitter elimination circuit in accordance with the result of detection bythe signal strength detection circuit.

What is claimed is:
 1. A receiver for receiving a signal transmitted by a time-division multi-access method, comprising: a signal strength detection circuit for outputting a reset signal when detecting a stronger signal strength of a signal transmitted by a multi-access method than a predetermined level at the start of a signal reception session in each reception cycle; a demodulator for demodulating the signal to output reproduced data and a reproduced clock; an oscillator for outputting an internal clock; a memory to which the reproduced data is written in synchronism with the reproduced clock and from which the reproduced data is read in synchronism with the internal clock; and an initialization circuit for initializing timing with which the reproduced data is written to and read from the memory in accordance with the reset signal, wherein the reset signal prevents a difference in timing with which the reproduced data is written at one frequency to and read at another frequency from the memory from becoming larger during reception.
 2. A receiver as claimed in claim 1, wherein the receiver is a portable terminal for receiving a signal transmitted by a time-division multi-access method from a base station of a Personal Handyphone System.
 3. A receiver as claimed in claim 1, wherein the memory is a FIFO memory.
 4. A receiving for receiving a signal transmitted by a time-division multi-access method that divides one frame into a plurality of communication slots in conducting communication, comprising: a signal strength detection circuit for detecting signal strength of a received RF signal; a comparison circuit for comparing an output fed from the signal strength detection circuit with a predetermined level at the start of a signal reception session in each frame and outputting a reset signal when the output is stronger than the predetermined level; a demodulation circuit for demodulating the received RF signal to obtain therefrom reproduced data and a reproduced clock; an oscillation circuit for generating an internal clock; a jitter elimination circuit for eliminating jitters from the reproduced data by writing the reproduced data to memory in synchronism with the reproduced clock and reading the reproduced data from the memory in synchronism with the internal clock; a processing circuit for decoding the reproduced data after jitter elimination; and an initialization circuit for initializing the jitter elimination circuit in accordance with the reset signal, wherein the reset signal prevents a difference in timing with which the reproduced data is written at one frequency to and read at another frequency from the memory from becoming larger during reception.
 5. A receiver as claimed in claim 4, wherein the initialization circuit initializes also the memory.
 6. A receiver as claimed in claim 4, wherein the initialization circuit initializes timing with which the reproduced data is written to and read from the memory.
 7. A receiver as claimed in claim 4, wherein the memory is a FIFO memory.
 8. A receiver as claimed in claim 4, wherein the receiver is a portable terminal for a Personal Handyphone System.
 9. A receiver for receiving a signal transmitted by a time-division multi-access method that divides one frame into a plurality of communication slots in conducting communication, comprising: an envelope detector for extracting an amplitude component from a received RF signal; an A/D converter for converting the amplitude component into a digital signal; a signal strength detection circuit for integrating the digital signal and outputting a resulting signal as a signal strength detection signal; a comparison circuit for comparing an output level of the signal strength detection signal with a predetermined level at a start of a signal reception session in a communication slot assigned to a receiver in each frame and outputting a reset signal when the output level is stronger than the predetermined level; a demodulation circuit for demodulating the received RF signal so as to output reproduced data and a reproduced clock; an oscillation circuit for generating an internal clock; a jitter elimination circuit that, when the reset signal is input, starts storing the reproduced data in a memory in synchronism with the reproduced clock and, after a predetermined delay time, starts reading and outputting the reproduced data sequentially from the memory in synchronism with the internal clock; and a processing circuit for decoding the reproduced data from the jitter elimination circuit.
 10. A receiver as claimed in claim 9, wherein the jitter elimination circuit further comprises: a FIFO memory having a plurality of stages; a counter for generating a first vector specification signal to specify a stage at which the reproduced data is written to and a second vector specification signal to specify a stage at which the reproduced data is read from; a delay circuit for outputting a first delay signal with which the second vector specification signal is generated after the first vector specification signal; a first latch circuit for feeding a second delay signal to the counter after receiving and latching the first delay signal so that the counter starts generating the second vector specification signal; and a second latch circuit for holding temporarily and outputting data read from the stage of the FIFO memory specified by the second vector specification signal. 